~ Pergamon Waf. Sci. T~ch. Vol. 33. No 4-5. pp. 227-232. 1996. Copyright ~ 1996 fA WQ. Pubhshed by Elsevier SCIence Ltd.

PH: S0273-1223(96)00235-1

Pnnled In Great Bmaln. All rights reserved. 0273-1223/96 $15'00 + 0'00

THE POTENTIAL IMPACT OF SOIL CARBON CONTENT ON GROUND WATER NITRATE CONTAMINATION

D. D. Adelman* and M. A. Tabidian**

* Water Resources Engineer. Adelman and Associates. 4535 Y St .• Lincoln. NE 68503-2344. USA *. Assistant Professor and Hydrogeologist. Cal. State University. Northridge. CA 91330-8266. USA

ABSTRACT

A potential buildup of nitrate in the ground water resources of the eastern Sand hills of Nebraska has been projected to occur due to the intensive use of nitrogen fertilizer on Imgaled cropland. A root-zone nitrate leaching study in this area revealed that soils with a high carbon concentration had minimal leaching compared to soils with lower concentrations. Soils high in carbon have an active population of demtnfying bacteria possibly causing denitrification and in turn reduction of nitrate leaching. Denitrifying bacteria are principally heterotrophic using soil organic carbon for both an energy and carbon source. The objective of thiS research was to interpret how root-zone denitrificallon affected nitrate leaching and ground water contamination by nitrate. A modified version of a solute transport model developed for the Eastern Sandhi lis was used to assess the nsk of nitrate contaminallon for combmallons of fertilizer and irrigation rates and for various soil carbon levels.

The first attempt was to make risk assessment with eight farm management practices for cells with increasingly greater carbon levels until only those cells with the greatest carbon level were kept in production. Results of this assessment showed that even with excessive fertilizer and irrigation rates. risk of nitrale leaching was reduced as the minimum carbon level was increased. However. since less cropland was leaching nitrate with each successIve risk calculation. the impact that root-lone denitrificatIon had in nitrate leaching reduction could not be definitively detennined. This prompted a model modification of the risk calculation procedure which kept all cropland in production and computed nitrate leachate risk for increasingly higher artificial carbon levels during successive risk calculations. Changmg carbon levels was still more detrimental on nitrate leaching rates than changing farm management practices. Copyright © 1996 IA WQ. Published by Elsevier Science Ltd.

KEYWORDS

Groundwater; Monte Carlo simulation; nitrate; nonpoint source pollution.

BACKGROUND

Agriculture related ground water quality problems in Nebraska such as groundwater nitrate contamination generally fall under the regulatory authority of regional natural resources districts (NRD). Some of these 23 natural resources districts have implemented a program to reduce groundwater nitrate contamination over a large area of their jurisdiction. Farmers within these management areas are required to follow "best management practices" emphasizing fertilizer application scheduling and requiring monitoring of certain

227

228 D. D. ADELMAN and M. A. TABIDIAN

sources of nitrate (CPNRD. 1984). For example. NRD nitrate management programs focus on minimizing nitrate leachate (NLeached) by scheduling fertilizer application time and requiring farmers to monitor the quantity of the nitrate sources (NResldual and NOroundwater) and application rates (NFenilizer)'

There are several sources and sinks of nitrate in the root-zone. The nitrate balance equation (Tanji et aI., 1979) consists of setting the nitrate sources equal to the nitrate sinks in the following relation:

NMineralized + NResldual + NRainfall + NOround water + NFenilizer = Nlmmobilized + NUptake + NDenitrified + NLeached (I)

where: NMinerahzed = Nitrate from mineralization of soil organic nitrogen. NResiduaJ = Nitrate from previous growing season. NRainfall = Nitrate from rainfall. NOround water = Nitrate from contaminated ground water used for irrigation, NFenilizer = Nitrate from commercial fertilizer and livestock manure, Nlmmobilized = Nitrate buildup in soil, NUptake = Nitrate taken up by the crop, NLeached = Nitrate leachate from the root-zone due to deep percolation of soil moisture, and NDenitrified = Nitrate lost from root-zone through denitrification.

In 1984, the Nebraska Natural Resources Commission contracted with the University of Nebraska-Lincoln Biological Systems Engineering Department to simulate cropland root-zone leaching of nitrate in the Eastern Sandhills of Nebraska (McIssac, 1984). In this study. nitrate leaching rates for different combinations of soil types, fertilizer and irrigation rates were calculated. Of the three soil types identified, one of them (Ipage) had a leaching rate substantially less than the leaching rates of the other two soils. Root-zone denitrification (NDenitrified term in Eq. I) was indicated to be the reason for the lower leaching rates.

Following the leaching study, a mixing cell solute transport model (Adelman and Dahab, 1993) was developed for the Eastern Sandhills unconfined aquifer where the leaching study was conducted. The model included a stochastic nitrate leaching component developed from the leaching study. The leaching component consisted of a multiple regression equation (r2 = 0.95) with the dependent variable being nitrate leaching rates and the independent variables consisting of fertilizer and irrigation rates, depth to water table, and soil carbon concentration. The stochastic input resulted a stochastic output for the areas with a risk of exceeding the U.s. EPA Maximum Contaminant Level (MCL) for nitrate (10 mgll NOrN). The objective of this research was to asses the role that root-zone denitrification could play in reducing the risk of excessive ground water nitrate in the Eastern Sandhills for different farm management practices and soil carbon levels.

SIMULATION PROCEDURES

The denitrification process is part of the nitrogen cycle consisting of the conversion of nitrate to eventually nitrogen gas:

where: N03 = Nitrate ion, NO = Nitrite ion,

(2)

NO = Nitric oxide gas, N20 = Nitrous oxide gas, and N2 = Molecular nitrogen gas.

Ground water nitrate contamination 229

There are five environmental factors influencing microbiological denitrification: acidity, moisture content, redox potential, temperature, and available carbon level. Denitrification is slower in an environment low in carbon, and, therefore, is enhanced by the addition of carbon-rich material. Denitrifying bacteria are generally heterotrophic using soil carbon for both a carbon and energy source.

Moisture content at the site of denitrification is a factor that influences the availability of oxygen or the redox potential. The effect of water is attributed to its role in governing the diffusion of oxygen to sites of denitrification. The lower the soil moisture at a denitrification site, the more oxygen diffuses to the site, making it aerobic and reduces the denitrification process. Usually, denitrification occurs at moisture levels above 60 percent of the water-holding capacity of a soil regardless of the carbohydrate supply, nitrate concentration, or pH. Above this moisture level, the rate and magnitude of denitrification are related directly to moisture content of the site (Alexander, 1977).

The stochastic nitrate leaching component mentioned above had two independent variables representing two of the five denitrification environmental influences. They are soil carbon level and soil moisture content For the computer model simulation and risk assessment of nitrate leaching, soil carbon concentration data available for an Eastern Sandhills solute transport model was used.

For the first risk assessment trial, all the area representing cropland with nitrate leaching was kept in production regardless of the soil carbon level of individual cells of the model. A farm management practice in terms of fertilizer and irrigation rates was selected. A random irrigation rate of the modeled area, using a normal random number generator, was generated to asses the risks for the first year. In this assessment. selected irrigation rates served as the mean value for the random irrigation rate. At the end of one-year risk assessment, the model computed a ground water nitrate concentration for each cell of the model. Another random irrigation rate was generated for the second simulation year with a groundwater nitrate concentration computed again for each cell. Then, after 40 years of simulation, it was determined which cells had a groundwater nitrate concentration in excess of the nitrate MeL. For a second risk assessment trial, a 40-year simulation followed for the selected farm management practice with all cropland kept in production regardless of the cells' soil carbon content A total of 50 repetitions of the simulation period resulted in generating a normal distribution of 50 ground water nitrate concentrations for each cell. If a cell had 40 excessive concentrations out of a possible 50, the risk of excessive nitrate would be «40150)*100=) 80 percent The overall model area weighted risk to a domestic consumer of model area groundwater was determined by weighing risk of individual cells according to number of farmsteads in each cell. Seven more farm management practices were selected and the model area Monte Carlo Simulation consisting of the 50 simulation period repetitions was repeated for each practice with all cropland kept in production regardless of the soil carbon level of individual model area cells.

Through a similar risk assessment but taking out of production those cells with a minimum soil carbon level. the overall ground water nitrate leaching risk for eight farm management were calculated.

Successive computer runs took out of production cells with cropland with the minimum carbon level from the previous run until the final run consisted of simulation of only those cells with the maximum soil carbon level. Additional computer runs consisted of keeping all cropland in production and computing ground water nitrate risk. In these runs, artificial soil carbon levels were increased for successive runs and risks for ground water contamination were calculated.

230 D. D. ADELMAN and M. A. T ABIDIAN

RESULTS AND DISCUSSION

Results from the computer runs where actual soil carbon levels were used in the simulations are shown in Tables I and 2. In this case, irrigation rates were expressed in terms of an irrigation replacement fraction (1RF) which is the decimal fraction of the crop irrigation requirement and risks were calculated. Risk results for four different fertilizer rates with an irrigation of 100%, or an IRF of 1.0, are shown in Table 1. Table 2 shows the results for an excessive irrigation rate, or an IRF of 1.5. These results illustrate that even with excessive fertilizer and irrigation rates, risk of nitrate leaching was reduced as the minimum simulated carbon level was increased. Since less cropland was leaching nitrate for each successive computer run, the actual impact that root-zone denitrification had in risk reduction could not be definitively determined.

Results from the computer runs where artificial soil carbon levels were simulated are shown in Tables 3 and 4. Risk results for four fertilizer rates and an IRF of 1.0 are shownin Table 3. Table 4 shows the results for an IRF of 1.5. Changing the selected carbon levels from a relatively minimal artificial level to an increasingly greater level while keeping all cropland in production was still more of an important factor with regard to reducing risk than changing farm management practices.

Table I. Risk For in situ soil carbon concentrations with efficient irrigation

Fertilizer Rate, Kq/Ha/Yr

Soil Carbon 100 150 200 265 Concentration

, mg/g

2.5 0.114 0.128 0.14 0.146

2.92 0.057 0.064 0.07 0.073

2.99 0.038 0.043 0.047 0.049

3.06 0.029 0.032 0.035 0.036 3.14 0.023 0.024 0.026 0.028

3.17 0.011 0.012 0.013 0.014

3.25 0.009 0.009 0.01 0.012

3.28 0.008 0.008 0.009 0.01

3.5 0.007 0.007 0.008 0.009

3.9 0.006 0.006 0.007 0.008

4.12 0.005 0.006 0.007 0.008 4.82 0.005 0.005 0.006 0.007 5.0 0.00 0.002 0.003 0.006

5.05 0.00 0.00 0.002 0.004 7.72 0.00 0.00 0.002 0.003

Ground water nitrate contamination 231

Table 2. Risk for in situ soil carbon concentrations with excessive irrigation

Fert1l1zer Rate, Kq/Ha/Yr

SoU Carbon 100 150 200 265 Concentration

, mg/g

2.5 0.139 0.146 0.146 0.146

2.92 0.064 0.07 0.073 0.073

2.99 0.042 0.046 0.048 0.049

3.06 0.032 0.035 0.036 0.037

3.14 0.024 0.026 0.028 0.028

3.17 0.012 0.013 0.014 0.014

3.25 0.009 0.01 0.012 0.012

3.28 0.008 0.009 0.01 0.01

3.5 0.007 0.008 0.009 0.009

3.9 0.006 0.007 0.008 0.008

4.12 0.006 0.007 0.008 0.008

4.82 0.005 0.006 0.007 0.007

5.0 0.002 0.003 0.006 0.006

5.05 0.00 0.002 0.004 0.004

7.72 0.00 0.002 0.003 0.003

Table 3. Risk for artificial soil carbon concentrations with efficient irrigation

Fertilizer Rate, Kg/Ha/yr

Carbon 100 150 200 265 Concentration

, mg/g

3 0.501 0.513 0.514 0.516

4 0.246 0.25 0.25 0.25

5 0.088 0.113 0.147 0.163

6 0.00 0.00 0.007 0.065

7 0.00 0.00 0.00 0.00

232 D. D. ADELMAN and M. A. T ABIDIAN

Table 4. Risk for artificial soil carbon concentrations with excessive irrigation.

Fertilizer Rate, Kq/Ha/yr

Carbon 100 150 200 265 Concentration

, mq/q

3 0.512 0.514 0.515 0.519

4 0.25 0.25 0.25 0.256

5 0.121 0.145 0.157 0.166

6 0.00 0.058 0.068 0.09 7 0.00 0.00 0.00 0.00

CONCLUSION

The results of this investigation clearly shows that if a reliable carbon source such as ethanol could be added to the root-zone, denitrification process in the root-zone will be enhanced and the risk for ground water nitrate contamination will be reduced significantly.

REFERENCES

Adelman. D. D. and Dahab, M. F. (1993). Risk Versus Economic Return in Managing Groundwater Nitrate Contamination, Wat. Sci. & Ttch.. 28(3-5), 55-63.

Alexander, M. (1977). Introduction to Soil Microbiology, John Wiley and Sons, New York, 467 pp. Central Platte Natural Resources District. (1985). Central Platte Natural Resources District Ground Water Management Plan,

Grand Island, Nebraska, 58 pp. McIsaac, G. F., Martin, D. L. and Schepers, J. S. (1984). Nitrate Leaching in Sandy Soils. American Society of Agricultural

Engineers Paper No. 84-2610, American Society of Agricultural Engineers, St. Joseph, Michigan, 20 pp. Tanji, K. K .• Broadbent, F. E •• Mehran. M. and Fried M. (1979). An Extended Version of a Conceptual Model for Evaluating

Annual Nitrogen Leaching Losses from Croplands, J. Envir. Qual .• 8(1), 114-120.